The hip joint (coxal articulation; coxofemoral articulation) is an enarthrodial or ball-and-socket joint, formed by the reception of the head of the femur into the cup-shaped cavity of the acetabulum.
The articular cartilage on the head of the femur, thicker at the center than at the circumference, covers the entire surface with the exception of the fovea capitis femoris, to which the ligamentum teres is attached; that on the acetabulum forms an incomplete marginal ring, the lunate surface. Within the lunate surface there is a circular depression devoid of cartilage, occupied in the fresh state by a mass of fat, covered by synovial membrane.
The ligaments of the joint are:
The articular capsule is strong and dense. Above, it is attached to the margin of the acetabulum 5 to 6 mm. beyond the glenoidal labrum behind; but in front, it is attached to the outer margin of the labrum, and, opposite to the notch where the margin of the cavity is deficient, it is connected to the transverse ligament, and by a few fibers to the edge of the obturator foramen. It surrounds the neck of the femur, and is attached, in front, to the intertrochanteric line; above, to the base of the neck; behind, to the neck, about 1.25 cm. above the intertrochanteric crest; below, to the lower part of the neck, close to the lesser trochanter. From its femoral attachment some of the fibers are reflected upward along the neck as longitudinal bands, termed retinacula. The capsule is much thicker at the upper and forepart of the joint, where the greatest amount of resistance is required; behind and below, it is thin and loose. It consists of two sets of fibers, circular and longitudinal. The circular fibers, zona orbicularis, are most abundant at the lower and back part of the capsule, and form a sling or collar around the neck of the femur. Anteriorly they blend with the deep surface of the iliofemoral ligament, and gain an attachment to the anterior inferior iliac spine. The longitudinal fibers are greatest in amount at the upper and front part of the capsule, where they are reinforced by distinct bands, or accessory ligaments, of which the most important is the iliofemoral ligament. The other accessory bands are known as the pubocapsular and the ischiocapsular ligaments. The external surface of the capsule is rough, covered by numerous muscles, and separated in front from the Psoas major and Iliacus by a bursa, which not infrequently communicates by a circular aperture with the cavity of the joint.
The synovial membrane is very extensive. Commencing at the margin of the cartilaginous surface of the head of the femur, it covers the portion of the neck which is contained within the joint; from the neck it is reflected on the internal surface of the capsule, covers both surfaces of the glenoidal labrum and the mass of fat contained in the depression at the bottom of the acetabulum, and ensheathes the ligamentum teres as far as the head of the femur. The joint cavity sometimes communicates through a hole in the capsule between the vertical band of the iliofemoral ligament and the pubocapsular ligament with a bursa situated on the deep surfaces of the Psoas major and Iliacus.
The muscles in relation with the joint are, in front, the Psoas major and Iliacus, separated from the capsule by a bursa; above, the reflected head of the Rectus femoris and Glutæus minimus, the latter being closely adherent to the capsule; medially, the Obturator externus and Pectineus; behind, the Piriformis, Gemellus superior, Obturator internus, Gemellus inferior, Obturator externus, and Quadratus femoris.
The arteries supplying the joint are derived from the obturator, medial femoral circumflex, and superior and inferior gluteals.
The nerves are articular branches from the sacral plexus, sciatic, obturator, accessory obturator, and a filament from the branch of the femoral supplying the Rectus femoris.
Movements.—The movements of the hip are very extensive, and consist of flexion, extension, adduction, abduction, circumduction, and rotation.
The length of the neck of the femur and its inclinations to the body of the bone have the effect of converting the angular movements of flexion, extension, adduction, and abduction partially into rotatory movements in the joint. Thus when the thigh is flexed or extended, the head of the femur, on account of the medial inclination of the neck, rotates within the acetabulum with only a slight amount of gliding to and fro. The forward slope of the neck similarly affects the movements of adduction and abduction. Conversely rotation of the thigh which is permitted by the upward inclination of the neck, is not a simple rotation of the head of the femur in the acetabulum, but is accompanied by a certain amount of gliding.
The hip-joint presents a very striking contrast to the shoulder-joint in the much more complete mechanical arrangements for its security and for the limitation of its movements. In the shoulder, as has been seen, the head of the humerus is not adapted at all in size to the glenoid cavity, and is hardly restrained in any of its ordinary movements by the capsule. In the hip-joint, on the contrary, the head of the femur is closely fitted to the acetabulum for an area extending over nearly half a sphere, and at the margin of the bony cup it is still more closely embraced by the glenoidal labrum, so that the head of the femur is held in its place by that ligament even when the fibers of the capsule have been quite divided. The iliofemoral ligament is the strongest of all the ligaments in the body, and is put on the stretch by any attempt to extend the femur beyond a straight line with the trunk. That is to say, this ligament is the chief agent in maintaining the erect position without muscular fatigue; for a vertical line passing through the center of gravity of the trunk falls behind the centers of rotation in the hip-joints, and therefore the pelvis tends to fall backward, but is prevented by the tension of the iliofemoral ligaments. The security of the joint may be provided for also by the two bones being directly united through the ligamentum teres; but it is doubtful whether this ligament has much influence upon the mechanism of the joint. When the knee is flexed, flexion of the hip-joint is arrested by the soft parts of the thigh and abdomen being brought into contact, and when the knee is extended, by the action of the hamstring muscles; extension is checked by the tension of the iliofemoral ligament; adduction by the thighs coming into contact; adduction with flexion by the lateral band of the iliofemoral ligament and the lateral part of the capsule; abduction by the medial band of the iliofemoral ligament and the pubocapsular ligament; rotation outward by the lateral band of the iliofemoral ligament; and rotation inward by the ischiocapsular ligament and the hinder part of the capsule. The muscles which flex the femur on the pelvis are the Psoas major, Iliacus, Rectus femoris, Sartorius, Pectineus, Adductores longus and brevis, and the anterior fibers of the Glutæi medius and minimus. Extension is mainly performed by the Glutæus maximus, assisted by the hamstring muscles and the ischial head of the Adductor magnus. The thigh is adducted by the Adductores magnus, longus, and brevis, the Pectineus, the Gracilis, and lower part of the Glutæus maximus, and abducted by the Glutæi medius and minimus, and the upper part of the Glutæus maximus. The muscles which rotate the thigh inward are the Glutæus minimus and the anterior fibers of the Glutæus medius, the Tensor fasciæ latæ and the Iliacus and Psoas major; while those which rotate it outward are the posterior fibers of the Glutæus medius, the Piriformis, Obturatores externus and internus, Gemelli superior and inferior, Quadratus femoris, Glutæus maximus, the Adductores longus, brevis, and magnus, the Pectineus, and the Sartorius.